Inkjet printers have been conventionally used to each print on a printing medium by scanning the printing medium with its head in which a plurality of nozzles for ejecting fine droplets of ink are arranged. Also well known are printers each of which prints while circulating ink between an ink tank storing ink and a head. In a printing apparatus shown in International Publication No. WO 98/19864, for example, circulation of ink is performed where ink is fed by a pump from an ink bottle storing ink to a print head in which a plurality of inkjet modules are arranged and the ink is returned from the print head to the ink bottle through another return line. In a printing apparatus shown in International Publication No. WO 00/38928, circulation of ink is performed where ink is supplied from a lower container positioned below a print head, being open to the air, to an upper container positioned above the print head, being open to the air, by a pump and the ink is returned from the upper container to the lower container through the print head.
In a printing apparatus in which circulation of ink is performed, even if air bubbles appear in the ink in a head (in a piezoelectric driving type head, for example, oscillation caused by high-speed driving of piezoelectricity sometimes causes air bubbles in ink), it is possible to return the air bubbles together with the ink to an ink tank and this suppresses the occurrence of a state where air bubbles move to nozzles of the head to temporarily block ejection of ink from the nozzles (the occurrence of the so-called missing nozzle). Further, circulation of ink can also resolve the precipitation of pigment which often occurs when white ink is used, and the like.
In a case where a plurality of heads are provided in the printing apparatus in which circulation of ink is performed, when respective pressures at nozzles in the plurality of heads are different, this causes landing positions on a printing medium and the like of ink with respect to the nozzles (i.e., relative landing positions of fine droplets of ink on the printing medium with respect to the positions of the nozzles in ejection of the fine droplets) to vary.